Hydrogen storage alloy material and process for producing the same

ABSTRACT

Disclosed is a hydrogen storage alloy material which is prepared by subjecting an amorphous alloy to a heat treatment in air or an oxygen atmosphere. The amorphous alloy has a composition, in atomic %, expressed by the following formula: Zr 100-a-b Pd a M b  (wherein 15≦a≦40, 0&lt;b≦10, and M is at least one metal selected from the group consisting of Pt, Au, Fe, Co and Ni). The Pd, the metal M and one or more compounds thereof are dispersed in a parent phase of ZrO 2  in the form of ultrafine particles. This alloy material exhibits a hydrogen storage amount of 2.5 weight % or more in a weight ratio relative to Pd contained in the material, and suited to a hydrogen storage/transportation medium. The alloy material can be produced by preparing a melt of a master alloy formed through a melting process, rapidly solidifying the melt at a cooling rate of 10 4  K/s or more to form the above amorphous alloy, and subjecting the amorphous alloy to an oxidizing heat treatment in air or an oxygen atmosphere at 250 to 350° C. to selectively oxidizing the alloy element of Zr so as to allow the hydrogen storage metal of Pd or a Pd compound to be dispersed in a parent phase of ZrO 2  in the form of nanoparticle-size ultrafine particles.

TECHNICAL FIELD

The present invention relates to a hydrogen storage alloy materialcontaining dispersed nanoparticles of a hydrogen storage metal, and aproduction method therefor, wherein a Zr—Pd amorphous alloy doped withat least one metal selected from the group of Pt, Au, Fe, Co and Ni isused as a precursor. In particular, the hydrogen storage alloy materialof the present invention is usable as a hydrogen storage containerexcellent in hydrogen absorption (storage)/desorption characteristics.

BACKGROUND ART

Heretofore, there have been known various hydrogen storage alloys, suchas an Mm (Misch metal)-Ni-based alloy and a Ti—V-based alloy. Inparticular, the Mm-Ni-based and Ti—V-based alloys are used as anelectrode material for butteries and a hydrogen storage material. Theseconventional hydrogen storage alloys have problems about insufficientcapacity when used as an electrode material for butteries, andexcessively large size in products when used as a hydrogen storagematerial, due to its low hydrogen storage amount.

As measures for increasing the hydrogen storage amount, various effortshave been made for the development of hydrogen storage alloy processingand the control of microstructure in alloys, such asnano-crystallization/amorphization of an alloy structure based on amechanical alloying process or a rapid liquid quenching process, as wellas researches on new alloys.

For example, an amorphous Mg—Ni-based hydrogen storage alloy capable ofabsorbing (storing) and desorbing hydrogen even in room temperatures hasbeen developed that is prepared by amorphizing an ordinary conventionalMg—Ni-based alloy through a mechanical alloying process (see, forexample, Patent Publications 1 and 2). There has also been known ahydrogen storage metal body consisting only of Pd metal fine particles(see Patent Publication 3). On the other hand, it is pointed out that afurther breakthrough is required to make the amorphous alloy-basedhydrogen storage alloy fit for practical use, because most of theamorphous alloy-based hydrogen storage alloys has no plateau.

Moreover, a production method based on a mechanical alloying process ora mechanical grinding process can provide final products only after longhours of powder mixing and alloy processing, which leads to a problemabout poor productivity.

[Patent Publication 1] Japanese Patent Laid-Open Publication No.11-061313

[Patent Publication 2] Japanese Patent Laid-Open Publication No.11-269572

[Patent Publication 3] Japanese Patent Laid-Open Publication No.04-311542

DISCLOSURE OF THE INVENTION

For the purpose of improving hydrogen storage characteristics, it hasbeen attempted to subject a hydrogen storage metal or alloy to amechanical alloying process or a mechanical grinding process so as toobtain a fine-grained polycrystalline structure. In view of the abovecircumstances, the inventors carried on various researches for obtaininga hydrogen storage alloy material having a microstructure with dispersednanoparticles of a hydrogen storage metal, and ability of moreefficiently absorbing and storing hydrogen.

As the result of the researches, through a process of preparing an alloyof Zr and a hydrogen storage metal of Pd, expressed by the followingformula: Zr_(100-x)Pd_(x) (x is a given atomic %, such as =35, 50 or60), oxidizing the alloy to selectively oxidize Zr in the alloy, theinventors obtained a material having a microstructure where ultrafinenanoparticles of the hydrogen storage metal Pd aggregated/formed from anamorphous state are dispersed in a parent phase of ZrO₂ (zirconia).

The inventors also found that this material has a hydrogen storageamount of 2.5 weight % or more in a weight ratio relative to Pdcontained in the material, which is far greater than a hydrogen storageamount (reported value) in a material consisting only of Pd in the sameweight as that of Pd contained in the material (“Journal of the JapanInstitute of Metals”, page 515, the Japan Institute of Metals, Oct. 1,2000).

Further, the inventors have achieved improvement in hydrogen desorptioncapacity of this hydrogen storage alloy.

Specifically, according to a first aspect of the present invention,there is provided a hydrogen storage alloy material prepared bysubjecting an amorphous alloy having a composition, in atomic %,expressed by the following formula: Zr_(100-a-b)Pd_(a)M_(b) (wherein15≦a≦40, 0>b≦10, and M is at least one metal selected from the groupconsisting of Pt, Au, Fe, Co and Ni), to a heat treatment in air or anoxygen atmosphere. The hydrogen storage alloy material has a structurewhere the Pd, the metal M and one or more compounds thereof aredispersed in a parent phase of ZrO₂ in the form of ultrafine particles.

The hydrogen storage alloy material of the present invention may exhibita hydrogen storage amount of 2.5 weight % or more in a weight ratiorelative to Pd contained in the material.

According to a second aspect of the present invention, there is provideda hydrogen storage/transportation container comprising a hydrogenstorage/transportation medium consisting of the hydrogen storage alloymaterial set forth in the first aspect of the present invention.

According to a third aspect of the present invention, there is provideda method for producing the hydrogen storage alloy material set forth inthe first aspect of the present invention, which comprises: preparing amelt of a master alloy formed through a melting process; rapidlysolidifying the melt at a cooling rate of 10⁴ K/s or more to form theamorphous alloy; and subjecting the amorphous alloy to an oxidizing heattreatment in air or an oxygen atmosphere at 250 to 350° C. toselectively oxidize the alloy element Zr so as to allow the Pd, themetal M and one or more compounds thereof to be dispersed in a parentphase of ZrO₂ in the form of nanoparticle-size ultrafine particles.

In the hydrogen storage alloy material set forth in the first aspect ofthe present invention, Pd (atomic %; “a”) is contained in the range of15 to 40 atomic %. If a content of Pd is less than 15 atomic % orgreater than 40 atomic %, hydrogen absorption/desorption capacities willbe lowered, resulting in loss of practicality. A hydrogen desorptioncapacity can be improved by adding 10 atomic % or more of the metal M toPd having a high hydrogen absorption capacity. Preferably, the metal Mis added in the range of 2 to 7 atomic %. If a total content of Pd andthe metal M (atomic %; “a+b”) is less than 15 atomic % or greater than50 atomic %, a content of Zr will be in the range of 85 to 50 atomic %,and thereby an alloy prepared by a rapid solidification process cannothave an amorphous structure. Moreover, the content of Pd deviating fromthe optimum range causes a change in microstructure. These lead tolowered hydrogen absorption/desorption amounts, resulting in loss ofpracticality. Preferably, a content of Zr is 65 atomic % or more in viewof facilitating the formation of an amorphous structure.

In the hydrogen storage alloy material of the present invention whereultrafine particles consisting of Pd, the metal M and one or morecompounds thereof and having a nanoparticle size of about 20 nm or lessare dispersed in ZrO₂, Pd contributes mainly to hydrogenabsorption/storage. A hydrogen storage amount in a weight ratio relativeto Pd contained in the material is 2.5 weight % or more, preferably 3weight % or more. In the material of the present invention, the parentphase of ZrO₂ essentially has no hydrogen storage characteristic. Thus,a hydrogen storage amount was evaluated based on only a weight of Pdcontained in the material, which is derived by subtracting a weight ofthe ZrO₂ from the total weight of the material. This value is the abovehydrogen storage amount in a weight ratio relative to Pd contained inthe material (Pd weight-based hydrogen storage amount).

In the present invention, the alloy prepared by a rapid solidificationprocess is used as a starting material. This makes it possible to forman amorphous structure in the material without segregation. Thisstarting material is oxidized to preferentially or selectively oxidizeZr which is one element of the Zr—Pd alloy, so that the element Pd in anamorphous state is aggregated to form ultrafine particles consisting ofthe Pd, the metal M and one or more compounds thereof and having ananoparticle size of about 20 nm or less and clean hetero-phaseboundaries without segregation, and the ultrafine particles aredispersed in the parent phase of ZnO₂. Thus, it is not desirable thatthe starting material exhibits crystalinity.

In the present invention, while a process for preparing the amorphousZr—Pd-based alloy as a starting material is not limited to a specificone, it is preferable to prepare the starting material by rapidlysolidifying a molten master alloy at a cooling rate of 10⁴ K/s or morethrough a liquid quenching process, such as a single-roll process, atwin-roll process, a gas atomization process or a melt extractionprocess.

The method for producing the hydrogen storage alloy material havingdispersed nanoparticle-size ultrafine hydrogen-storage-metal particlesaccording to the first aspect of the present invention will be describedbelow.

Firstly, a master alloy is formed through a melting process in such asmanner as to have an intended alloy composition. It is preferable toperform the melting process in an arc melting furnace filled with aninert atmosphere, such as argon. Then, the master alloy is re-molten,and the obtained melt is rapidly solidified at a cooling rate of 10⁴ K/sor more to form a rapidly-solidified alloy.

This rapid solidification process at a cooling rate of 10⁴ K/s or moremay be achieved by using one of various conventional processes, such asa single-roll process, a twin-roll process, a gas atomization process ora rotary submerged spraying process. In the present invention, it ispreferable to use the single-roll process having an advantage of beingable to relatively easily control a cooling rate. A cooling rate of lessthan 10⁴ K/s causes difficulty in forming an amorphous structure.

Then, the obtained starting material consisting of the amorphous alloyin the form of a foil, powder or wire is oxidized in air or an oxygenatmosphere at about 250 or 350° C. for about 24 hours. The heatingprocess is not limited to a specific one, but any suitable heatingprocess excellent in productivity may be used. In view of allowing onlyZr in the material to be selectively oxidized while suppressingexcessive oxidization of Pd and other elements, it is undesirable toheat the material up to a high temperature of 400° C. or more. If theheating temperature is less than 250° C., a sufficient oxidizationcannot be obtained.

The state of metal particles precipitated in the parent phase of ZrO₂ isvaried depending on a content of Pd and a kind and content of the metalM. For example, when a content of the metal M is reduced, (1) Pd-Mparticles and surplus-Pd particles or (2) an oxide thereof will beprecipitated in the ZrO₂, or (3) Pd and M will be separatelyprecipitated in the ZrO₂. When a content of the metal M is increased,(1) Pd-M alloy particles (compound: intermetallic) and surplus-Mparticles or (2) an oxide thereof will be precipitated in ZrO₂, or (3)Pd and M will be separately precipitated in ZrO₂ (if Pd and M are hardlyincorporated into each other as a solid solution.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be specifically described in connectionwith Inventive and Comparative Examples.

Example 1 & Comparative Examples 1 and 2

Each of master alloys having various compositions as shown in Table 1was formed through a melting process using an arc melting furnace in anargon atmosphere. Then, the master alloy was subjected to a single-rollprocess to form a flat-shaped rapidly-solidified thin strip. Morespecifically, in the single-roll process, the master alloy was moltenusing a quartz nozzle under an argon atmosphere, and the molten masteralloy was injected at 0.3 kg/cm² onto a copper roll having a diameter of20 cm and rotating at 4000 rpm, through a quartz nozzle having anozzle-hole diameter of 0.3 mm, so that the molten master alloy wasrapidly solidified and formed as an amorphous alloy foil having a width1 mm and a thickness of 20 μm. Then, this amorphous alloy foil wasoxidized in air or an oxygen atmosphere at 280 to 350° C. for about 24hours. The obtained alloy was crushed or powdered into a size of about30 μm, and subjected to a measurement of hydrogen storage amount undertemperatures of 50° C. and 150° C.

The measurement was performed using a Sieverts-type PCT-characteristicsmeasuring apparatus (made by Suzuki Shokan Co., Ltd., Japan) under thecondition of hydrogen pressurization of up to 5 MPa. Further, before anoperation for hydrogen absorption, a phase identification using an X-raydiffraction process was performed to determine whether a crystal phasewas precipitated.

In Table 1, Pd weight-based hydrogen storage amount in InventiveExamples 1 and 2 means a hydrogen storage amount in a weight ratiorelative to Pd+Ni contained in the material, and Pd weight-basedhydrogen storage amount in Comparative Example 5 means a hydrogenstorage amount in a weight ratio relative to Au contained in thematerial. TABLE 1 Measurement Hydrogen Pd weight- Pd weight- temperaturestorage based based Composition of hydrogen amount of hydrogen hydrogenof starting storage entire storage desorption Phase of materialcharacteristics material amount amount starting (at. %) (° C.) (wt. %)(wt. %) (wt. %) Structure material Inventive Zr₆₅Pd₃₀Ni₅ 150 0.71 2.301.21 Pd nanoparticles + amorphous Example 1 ZrO₂ Inventive Zr₆₅Pd₃₀Ni₅50 0.78 2.51 1.57 Pd—Ni ally amorphous Example 2 nanoparticles + ZrO₂Comparative Zr₆₅Pd₃₅ 150 0.71 2.19 0.54 Pd nanoparticles + amorphousExample 1 ZrO₂ Comparative Zr₆₅Pd₃₅ 50 0.84 2.58 1.33 Pd nanoparticles +amorphous Example 2 ZrO₂ Comparative Zr₅₀Pd₅₀ 150 0.45 0.95 0.32 Pdcoarse particles + crystalline Example 3 ZrO₂ Comparative Zr₅₀Pd₅₀ 500.59 1.26 0.61 Pd coarse particles + crystalline Example 4 ZrO₂Comparative Zr₇₀Pd₃₀ 150 0.44 0.09 0.09 Au nanoparticles + amorphousExample 5 ZrO₂ Comparative Pd (reported 150 0.65 0.65 0.65 Pdcrystalline Example 6 value) (polycrystalline structure of coarseparticles) Comparative Pd (reported 50 0.69 0.69 0.69 Pd crystallineExample 7 value) (polycrystalline structure of coarse particles)

As is clear from Table 1, in Inventive Examples 1 and 2 using a Zr—Pd—Niamorphous alloy as a starting material, a hydrogen storage amount of theentire material is 0.7 weight % or more. In contrast, ComparativeExamples 3 to 5 having a composition deviating from the compositionrange of the present invention (each of Comparative Examples 3 and 4contains Pd in an amount beyond the range allowing the starting materialto be formed as an amorphous structure, and Comparative Example 5contains Au having no hydrogen storage capacity in place of Pd) exhibitsa hydrogen storage amount inferior to those of Inventive Examples 1 and2.

As mentioned above, the parent phase of ZrO₂ in the hydrogen storagealloy material of the present invention essentially has no hydrogenstorage characteristic, and thereby a value evaluated based on only aweight of Pd contained in the material, which is derived by subtractinga weight of the ZrO₂ from the total weight of the material, is ahydrogen storage amount in a weight ratio relative to Pd contained inthe material (Pd weight-based hydrogen storage amount). Referring toTable 1 for this value, while the Pd weight-based hydrogen storageamount is 2 weight % or more in each of Inventive Examples 1 and 2 andComparative Examples 1 and 2, it is less than 1.5 weight % in each ofComparative Examples 3 to 5. Comparing with a hydrogen storage amount(reported value) in each of Comparative Examples 6 and 7 consisting onlyof Pd, each of Inventive Examples 1 and 2 has an absorption efficiency 3to 4 times greater than those of them. This shows that themicrostructure with dispersed hydrogen-storage-metal nanoparticlesproduced by the method of the present invention drastically enhances anoriginal hydrogen storage capacity of the hydrogen storage metal.

In addition, each of Inventive Examples 1 and 2 using the startingmaterial prepared by substituting Ni as the metal M for 5 atomic % of Pdin the total Pd content of Comparative Example 1 or 2 has approximatelythe same hydrogen storage amount as those of Comparative Example 1 and2. However, when a difference between a maximum hydrogen storage amountunder a maximum equilibrium hydrogen pressure of about 4.5 MPa in theaforementioned measuring apparatus, and a residual hydrogen storageamount after hydrogen desorption is calculated as a hydrogen desorptionamount in a weight ratio relative to Pd contained in the material (Pdweight-based hydrogen desorption amount), the Pd weight-based hydrogendesorption amount in each of Inventive Examples 1 and 2 is superior tothose of Comparative Example 1 and 2. This verifies that a hydrogenstorage alloy having higher practicality can be produced by adding themetal M.

INDUSTRIAL APPLICABILITY

The hydrogen storage alloy of the present invention using as a precursorthe Zr—Pd-M (M is at least one metal selected from the group of Pt, Au,Fe, Co and Ni) amorphous alloy has excellent hydrogen absorption(storage)/desorption efficiency. Thus, this hydrogen storage alloy canbe suitably used as a hydrogen storage material in various fields,particularly a stationary hydrogen storage facility, and can serve as aguideline for a structural design of a material capable of efficientlyincorporating hydrogen therein. Further, the production method of thepresent invention makes it possible to prepare the precursor comprisingthe Zr—Pd-M (M is at least one metal selected from the group of Pt, Au,Fe, Co and Ni) amorphous alloy using a single-roll process allowing aserial production and oxidize the precursor so as to providehighly-efficient hydrogen storage ally, in a simple and easy manner.

1. A hydrogen storage alloy material prepared by subjecting an amorphousalloy to a heat treatment in air or an oxygen atmosphere, said amorphousalloy having a composition, in atomic %, expressed by the followingformula: Zr_(100-a-b)Pd_(a)M_(b) (wherein 15≦a≦40, 0<b≦10, and M is atleast one metal selected from the group consisting of Pt, Au, Fe, Co andNi), wherein said hydrogen storage alloy material has a structure wheresaid Pd, said metal M and one or more compounds thereof are dispersed ina parent phase of ZrO₂ in the form of ultrafine particles.
 2. Thehydrogen storage alloy material as defined in claim 1, which exhibits ahydrogen storage amount of 2.5 weight % or more in a weight ratiorelative to Pd contained in said hydrogen storage alloy material.
 3. Ahydrogen storage/transportation container comprising a hydrogenstorage/transportation medium consisting of the hydrogen storage alloymaterial as defined in claim 1 or
 2. 4. A method for producing thehydrogen storage alloy material as defined in claim 1, comprising:preparing a melt of a master alloy formed through a melting process;rapidly solidifying said melt at a cooling rate of 10⁴ K/s or more toform said amorphous alloy; and subjecting said amorphous alloy to anoxidizing heat treatment in air or an oxygen atmosphere at 250 to 350°C. to selectively oxidize said alloy element Zr so as to allow said Pd,said metal M and one or more compounds thereof to be dispersed in aparent phase of ZrO₂ in the form of nanoparticle-size ultrafineparticles.